Field of the Invention
The invention relates to an apparatus and a method for detecting the thickness of a brake lining.
Such an apparatus and method serve to monitor the state of the brake linings on the wheel brakes of a motor vehicle. Since the brake linings in all friction brakes are subjected to constant wear, information regarding the thickness of the linings is very important for the functioning of the brake. The brake linings can be monitored from such information to ensure that they maintain a minimum thickness which is critical for functioning and safety.
In an apparatus for monitoring a brake lining, which is known from German Published, Non-Prosecuted Patent Application DE 38 18 877 A1, a wear sensor has a plurality of electric conductors disposed at a distance from one another which are worn down with the brake lining. The number of conductors which are exposed in the process and have a short circuit to the chassis is a measure of the wear of the brake lining. Each short circuit to the chassis trips a fuse and reduces a monitoring current. German Published, Non-Prosecuted Patent Application DE 42 31 107 A1 discloses a sensor for determining the lining thickness and at the same time the temperature of brake linings. In that sensor an electric resistor which is disposed on the brake lining has a resistance value that is a function of its length and an electric resistance that has the same chassis ground is coupled thereto. Such a sensor is relatively complicated, changes to the brake linings are necessary and the resistor material must correspond to the lining material in terms of abrasion behavior and withstand the high temperatures occurring in the friction surface without showing signs of aging.
The increasing requirements currently imposed on modern brake systems of motor vehicles, such as anti-lock devices, travel-stability control systems, drive-slip control devices or traction control devices, necessitate braking intervention on selective wheels. In conventional brake systems, braking intervention on selective wheels is made possible with additional hydraulic pumps and solenoid valves. However, such brake systems are complicated and are not without problems. Braking intervention on selective wheels is achieved in a smoother and less complicated manner with brake systems in which each wheel is allocated an electrically controlled wheel-brake actuator that is integrated in the wheel-brake caliper and in which the requisite braking moment is produced with a brake motor. In a brake-actuating configuration known from German Published, Non-Prosecuted Patent Application DE 42 29 042 A1, an electric motor drives a drive piston through a reduction gear and a device for transforming the rotary motion of the electric motor into a linear motion. The pressure produced in a fluid chamber in that case acts on a brake-lining pressure piston. Such a brake system is also designated below as a "brake-by-wire" system or brake.